Highly compressed matter in which the normal atomic structure has
broken down and which, because of quantum-mechanical effects, exerts a
pressure that is independent of temperature. Bodies with masses less than
→ Chandrasekhar's limit (1.4 solar masses) are supported by
electron → degeneracy pressure and have densities of
about 106 kg/m3. In collapsed stars of mass above 1.4 solar masses,
gravity will overwhelm electron degeneracy and further collapse ensues.
Electrons combine with protons to form neutrons, so producing a
→ neutron star. Because neutrons, like electrons, are
→ fermions and therefore subject to the
→ Pauli exclusion principle, at high enough densities,
about 1014 kg/m3, neutron degeneracy pressure prevents
further collapse of the star. For masses larger than 2-3 solar masses, even neutron
degeneracy cannot prevent further collapse, and a → black hole is formed.